In vehicles with downhill speed control systems, a cruise control offset may be provided when downhill speed control is active. This allows the vehicle speed to exceed the cruise control set speed by the specified offset amount on a downhill grade. An engine retarder system may be used in such vehicles to automatically apply retarding force to modulate vehicle speed in an attempt to stay within the fixed cruise control offset while the downhill speed control system is active.
Separately, the operator also may be provided with the ability to manually adjust an engine retarder demand setting to select a desired level of auxiliary braking. An increase in engine retarder demand typically leads to decreased vehicle speed as engine retarder torque increases. This provides an additional level of control for the operator.
However, in vehicles with downhill speed control systems, the vehicle does not react to an operator's manual adjustment to engine retarder demand while the downhill speed control system is active. Although it is possible to provide a separate input device, such as a button or switch, to manually adjust downhill speed offsets, this adds complexity for the operator and reduces the amount of usable space for operator controls in the cab of the vehicle.